1. Field of the Invention
The present invention relates to a variable wavelength dispersion compensator for variably compensating wavelength dispersion in an optical fiber communications system.
2. Description of the Related Art
An optical fiber communications system generally has a problem that the distortion of a transmission waveform due to optical fiber wavelength dispersion (chromatic dispersion) degrades signal quality. Therefore, the wavelength dispersion must be compensated.
For a dispersion compensation method, a method for restoring waveform distortion by inserting a device having a dispersion characteristic the opposite of an optical fiber (dispersion compensation fiber) in a transmission line is used. Furthermore, variable dispersion compensators have been developed which incorporate a chromatic dispersion generation device (VIPA—“virtually imaged phased array”) and a light-returning device (non-spherical mirror) in order to cope with the change of the dispersion characteristic due to the temperature, the pressure and the like of an optical fiber (Japanese Patent Applications 10-534450 and 11-513133).
FIG. 1 shows the basic configuration of a variable dispersion compensator using a VIPA.
Beams inputted from a fiber 10 are collected in a form of a line or dots by a lens 11 and are inputted to a VIPA 12. The VIPA is a transparent parallel plate on both sides of which a reflection film is formed. Although one reflection film has a reflectance of 100%, the other has a reflectance of less than 100%, and typically of 95%. Therefore, beams inputted to the VIPA 12 are repeatedly reflected between these reflection films and some of the beams are repeatedly outputted to the outside at one time from a surface with a low reflectance. Since the beams are outputted to the outside at each reflection, the beams have phase differences between each other. Therefore, if the beams interfere with each other, beams with a prescribed wavelength are formed into luminous flux that propagates in a prescribed direction. Thus, the VIPA 12 is a device for generating a plurality of pieces of luminous flux that propagates in different directions depending on the wavelengths.
The outputted beams are collected at a lens 13 and are reflected on a non-spherical mirror 14. In this case, as shown by dotted lines in FIG. 1, if attention is focused on one beam of the luminous flux, by reflecting a specific beam on the non-spherical mirror and by changing the input position after return from the output position from the VIPA when returning the beam to the VIPA 12, the distance between the lens 11 and fiber 10 that the beam propagates can be changed. Specifically, the propagation distance can be extended, and propagation delay can be applied to the beam. If a plurality of beams with different wavelengths take different routes, the respective propagation delay of the beams can be changed by a wavelength, and wavelength dispersion can be generated, accordingly. If the wavelength dispersion of an optical fiber is compensated, a reciprocal dispersion having a reverse characteristic of canceling the wavelength dispersion of the beam is applied to the beam.
This compensator has a characteristic of freely changing a compensation amount by moving the non-spherical mirror depending on a dispersion value. The non-spherical mirror has a gradation structure, such as a concave surface and a convex surface.
FIG. 2 shows a non-spherical mirror.
This non-spherical mirror is located on a moving stage. If the mirror is moved in the direction of an arrow shown in FIG. 2, the shape of the light input position of the mirror changes. Therefore, a plurality of different chromatic dispersions (wavelength dispersions) can be generated.
FIG. 3 shows the wavelength dispersion and signal degradation of a transmission line, and the compensation.
For example, as shown in FIG. 3, if an input pulse (1) is transmitted from a transmitter and is received by a receiver through an optical fiber, the pulse width of an output pulse (2) is expanded by wavelength dispersion and the pulse is distorted. In this case, if a variable dispersion compensator (hereinafter a VIPA, for example) is inserted and reciprocal dispersion is given to the output pulse (2), the distortion of the pulse can be compensated. Therefore, the receiver can receive a pulse without distortion (3).
FIG. 4 shows dispersion to be compensated by a VIPA.
If the wavelength of a pulse and the dispersion of an output pulse (3) are assumed to be λ0 and 100 ps/nm, respectively, the relationship between the wavelength and dispersion becomes as shown in FIG. 4. In this case, dispersion compensation by a VIPA means the total dispersion amount is reduced to 0 ps/nm. Thus, a post-compensation pulse of 0 ps/nm is generated. Thus, the VIPA reduces the total dispersion amount to zero by shifting the dispersion amount that a beam suffers from the propagation through the optical fiber upward or downward (reciprocal dispersion).
<Problem No. 1>
According to the conventional method described above, the stage must be moved depending on a dispersion compensation amount. Therefore, if the compensation range is extended, the non-spherical mirror must be made longer and a movement amount also increases. However, since the increase in a stage movement amount greatly affects the accuracy of the stage movement, dispersion cannot be accurately compensated, which is a problem.
Once a non-spherical mirror is designed, the mirror can compensate for only a specific band. Therefore, in order to compensate for a new band, a new non-spherical mirror must be designed.
<Problem No. 2>
Although the conventional method gives a pulse with reciprocal dispersion, a case where this method is applied to a WDM beam is studied.
FIG. 5 shows a case where the conventional wavelength dispersion method is applied to a WDM beam.
In this case, it is assumed that there are three waves (λ1<λ0<λ2). As shown in FIG. 5, λ1, λ0, and λ2 take different dispersion values depending on the wavelengths, that is, a dispersion slope in an optical fiber (curve 1). In this case, if the dispersion is shifted so that the dispersion value of λ0 becomes zero in a VIPA, as shown in FIG. 4, the dispersion values of λ1 and λ2 do not become zero. Since the VIPA shifts the curve by a specific dispersion amount throughout the entire wavelength, the VIPA simply shifts curve 1 upward or downward. Therefore, it is impossible to simultaneously reduce all the dispersion values of λ1, λ0 and λ2 to zero, which is also a problem.